1. Field of the Invention
The invention concerns: a method to create a magnetic resonance (MR) magnitude image data set and a phase image data set of an examination subject; an MR system and a computer-readable data storage medium for implementing such a method in a computerized processor.
2. Description of the Prior Art
One application field of magnetic resonance systems is the monitoring of medical procedures or treatments, for example thermotherapy, in which the temperature in tissue (for example tumor cells) is specifically increased, which ideally leads to cell death or (given a smaller temperature increase) allows the cells to become more sensitive to accompanying therapy measures such as chemotherapy or radiation therapy. A cooling is likewise possible in a treatment known as cryotherapy. Particularly in the case of ablation of tumor tissue (for example by means if high-intensity focused ultrasound), magnetic resonance systems are increasingly commonly used for 3-dimensional temperature imaging in order to show the temperatures prevailing in the treated area with optimally high precision and high time resolution during a treatment. Optical monitoring during the treatment should not only show the temperature of the heated tissue with time and spatial resolution, but also it should be possible to establish a relationship of the measured temperature images with the anatomy of the examined person.
One possibility to show temperature changes with the use of magnetic resonance tomography is the proton resonance frequency method that is based on the temperature dependency of the resonant frequency of protons. The phase information of the MR signal that is obtained from gradient echo signals is used in order to conclude a temperature change from the difference of two phase images. The temperature information can be shown with spatial resolution through the presentation of phase difference images. The relation between phase change and a temperature changes is as follows:φ=γB0TEαT, or Δφ=γB0TEαT,  (1)wherein B0 is the basic magnetic field strength, γ is the gyromagnetic ratio, TE is the echo time, α is the temperature dependency of the resonance frequency (which is −0.01 ppm/° C.) and ΔT is the temperature change. Since the acquired MR data that are used to create the phase differences are plagued with noise, the temperature changes determined from them can be determined only with a limited precision. As can be recognized from the above equation, it would be advantageous to select the echo time to be as long as possible in order to induce an optimally large phase change; however, this extends the acquisition time and reduces the signal-to-noise ratio overall since the signal level overall decreases with increasing echo time due to the T2* decay of the magnetization.